This invention relates to apparatus suitable for freezing food.
Nowadays, liquid nitrogen and, to a lesser extent, liquid carbon dioxide, are widely used to freeze food. A common method of employing the liquid nitrogen or liquid carbon dioxide to freeze food is to advance the food to be frozen on a convenyor through a tunnel into which the liquefied gas is sprayed. Heat exchange takes place between the liquefied gas and the food and also between the food and the cold gas formed as a result of the vaporisation of the liquid. Fans are typically employed to promote the heat exchange between the cold gas and the food. Food freezing tunnels are, for example, described in U.K. Pat. specification No. 2 076 952 B and U.S. Pat. No. 4,171,625.
Typically, conventional food freezing tunnels may be from 20 to 60 feet long and from two to four feet wide. In order to achieve good utilisation of the refrigerative capacity of the liquid nitrogen or liquid carbon dioxide, it is necessary to ensure that the tunnel carcass is thermally-insulated. Although the most effective insulation for use at cryogenic temperatures is vacuum-insulation, it has not proven economically feasible to employ vacuum-insulation in food freezing tunnels. Accordingly it is conventional to employ an insulating material such as foamed polyurethane in the carcass of a freezing tunnel, and, typically the walls of the tunnel are made from inner and outer spaced stainless steel skins with the space therebetween packed with suitable termally-insulating material such as foamed polyurethane. Notwithstanding the presence of this thermal insulation, there is still typically an appreciable inleak of heat into the tunnel directly through the walls thereof. Such in leak of heat, we have found, tends to be enhanced by a number of phenomena. First, the spray header or other means of injecting the liquid nitrogen or liquid carbon dioxide into the tunnel may spray liquefied gas directly at the side walls of the tunnel in the event that the spray header is badly designed or the tunnel is particularly narrow. Second, it is inevitable that the fans employed in the tunnel will direct some flow of cold gas in the direction of the side walls of the tunnel carcass. Third, the liquid nitrogen sometimes passes through the conveyor belt and can collect on the floor of the tunnel itself. Fourth, direct inpingement of liquid nitrogen or its cold vapour on elastomeric seals employed to seal access openings to the tunnel can cause failure of the seal. Moreover in the kind of freezing tunnel in which the roof of the tunnel may be moved out of engagement with the rest of the tunnel housing, or vice versa, elastomeric seals between the two sections of the tunnel may become so cold that a considerable quantity of atmospheric moisture freezes on its outer surfaces with the result that the tunnel sections become stuck together and considerable force needs to be applied to separate them again.
It is an aim of the present invention to provide an improved tunnel which ameliorates the above-mentioned problems by inhibiting contact between the liquefied gas or its cold vapour and the sides of the tunnel.